Chemistry Reference
In-Depth Information
TABLE 8B.33. Asymmetric Allylations Using L1 and Bisoxazoline Ligands L45-L47
MeO
2
C
CO
2
Me
+
Na CH(CO
2
Me)
2
CO
2
Me
R
OCO
2
Me
R
(C
2
H
5
CN)
3
Mo(CO)
3
(10 mol %),
L*
, THF, 70°C
R
CO
2
Me
b
l
Entry
R
Ligand
t (day)
Yield (%)
b
:
l
ee (%)
1
P h
L43a
3 h
70
49:1
99 (
R
)
2
P h
L45
0.5
86
14:1
99 (
R
)
3
P h
L46b
1
83
6:1
98 (
R
)
4
P h
L46b
1
83
6:1
84 (
R
)
5
P r
L43a
1.5
80
8:1
98 ( − )
6
P r
L47b
1.5
84
8:1
98 (
−
)
7
P r
L47c
1.5
83
8:1
97 (
−
)
8
P r
L46b
2
69
2:1
96 (+)
9
P r
L46c
2.5
65
2:1
86 (+)
10
Me
L43a
1
85
5:1
94 (
R
)
11
Me
L45
1
88
1.5:1
94 (
R
)
12
Me
L46a
5
73
5:1
74 (
R
)
13
Me
L47a
3
76
7:1
85 (
S
)
14
Me
L46b
1
81
9:1
97 (
R
)
15
Me
L47b
2
80
11:1
96 (
S
)
16
Me
L46c
1.5
81
9:1
95 (
R
)
17
Me
L47c
1
86
7:1
92 (
S
)
18
PhO
L46b
2
79
20:1
98 (
−
)
Next, the alkyl-substituted substrates were investigated, which are more critical with
respect to regio- and enantioselectivity. As expected, the (
E
) - hexenyl methyl carbonate
was less reactive than the corresponding phenyl-substituted analogue and required reac-
tion times of 1.5-2 days. Ligands
L43a
,
L47b
, and
L47c
all gave an 8:1 ratio of
b
:
l
with
ees around 98% (entries 5-7). In contrast, the diastereomeric ligands
L46b
and
L46c
,
derived from the enantiomeric 1,3-diaminocyclohexane, gave a lower
b
:
l
ratio, and in
case of
L46c
, a signifi cantly lower ee (entries 8 and 9). These ligands also induced the
opposite confi guration in the branched alkylation product, indicating that the enantiose-
lectivity is mainly controlled by the diaminocyclohexane unit.
All ligands were also applied to reactions of crotyl methyl carbonate. The ligand
L45
gave the same high yield and ee as
L43a
, although the regioselectivity was lower (entries
10 and 11). The phenyl-substituted ligands
L46a
and
L47a
gave a higher branched/linear
ratio, although at the expense of lower ees (entries 12 and 13). The best results were
obtained with the
n
- propyl derivatives
L46b
and
L47b
, which were slightly superior to
the isopropyl ligand
L46c
and
L47c
, and better than
L43a
(entries 14 - 17). Interestingly,
the
t
-butyl-substituted ligand provided racemic material. Several other substituted allylic
substrates were tested, including alkoxy- and phenoxy-substituted ones. For example,
the phenoxy-substituted allylic carbonate gave branched/linear ratios
>
20:1 with all
ligands and ees
>
93. The best results were obtained with
L46b
(entry 18).
